Image-forming devices are frequently used to form images on media, such as paper and other types of media. Image-forming devices include laser printers, inkjet printers, and other types of printers and other types of image-forming devices. Media is commonly moved through an image-forming device as the device forms the image on the media. The image-forming mechanism of the device, such as an inkjet-printing mechanism, may move in a direction perpendicular to that in which the media moves through the image-forming device. Alternatively, the image-forming mechanism may remain in place while the media moves past it.
For high-quality image formation, the movement of the media through an image-forming device is desirably precisely controlled. If the media moves more than intended, there may be gaps in the resulting image formed on the media, whereas if the media moves less than intended, there may be areas of overlap in the resulting image. More generally, the image quality of the printed output may be reduced. A media-advance sensor can be used to measure media advancement. However, high-quality media-advance sensors can be expensive, rendering their inclusion in lower-cost and mid-cost image-forming devices prohibitive. Less accurate and less costly sensors may be used, but they may provide less than desired sensing capabilities.
Additionally, if the media moves through the image-forming mechanism in a skewed manner other problems may arise. For example, the lateral sides of the media may impact against the sides of the image-forming mechanism, damaging the media and/or causing a media jam. In some existing inkjet printers, an optical sensor mounted to a scanning printer carriage may be used to measure the position of the lateral sides of the print media relative to the scan axis of the printer mechanism. In this manner media skew may in certain situations be determined. This solution suffers from the drawback of requiring the carriage to repeatedly pass over the edges of the print media so that this measurement may be made. This means that during times when this does not happen, the media skew is not measured. Examples of such times include: during large media feeds; and, when the carriage travel is optimised to move only the distance required to print the current swath, and the swath does extend near to the lateral edges of the media.